LEDs are rapidly replacing incandescent bulbs, fluorescent bulbs, and other types of light sources due to their efficiency, small size, high reliability, and selectable color emission. A typical forward voltage drop for a high power LED is about 3-4 volts. The brightness of an LED is controlled by the current through the LED, which ranges from a few milliamps to an amp or more, depending on the type of LED. For this reason, LED drivers typically include some means to control the current.
In applications where high brightness is needed, many LEDs are used. It is common to connect LEDs in series, since the current through all the LEDs in series will be the same. Redundant drivers and strings may also be incorporated for added reliability and increased brightness. Serial strings of LEDs may also be connected in parallel so as to limit the required driving voltage level and provide redundancy.
In some applications, there may be 25 or more LEDs connected in series, requiring a driving voltage of about 90 volts (about 25×3.5 volts). Such high voltages require components with a breakdown voltage well in excess of 90 volts.
FIG. 1 is a typical LED driver 10 with a boost topology that drives one or more strings of multiple LEDs D1-D4.
The LEDs' color is slightly dependent on the magnitude of the forward current. Once the forward current is fixed, it should be not changed in order to avoid color shift.
A series resistor 14 is connected in-between the LEDs D1-D4 and ground. The feedback voltage (VFB) at the resistor R14 is the LED current times the resistance of resistor R14.
A pulse-width modulation (PWM) brightness control signal 16 has a relatively low frequency (e.g., 100 Hz-1000 Hz) and effectively enables the boost regulator at the LF PWM duty cycle. The brightness control signal 16 is divided by resistors R18 and R19. The magnitude of the high state PWM voltage is VBRIGHT. The divided voltage is VBRIGHT×R18/(R19+R18). The divided voltage and VFB are applied to inputs of an op amp 22, acting as an error amplifier. The feedback loop of the boost regulator causes the output voltage (VOUT) of the regulator, applied to the LED string, to be of a level to maintain VFB at VBRIGHT×R18/(R19+R18). Therefore, the forward current ILED through the LEDs when VBRIGHT is high is VBRIGHT×R18/((R19+R18)*R14).
A capacitor 24 at the output of the op amp 22 provides a relatively stable voltage while VBRIGHT is high. The voltage at capacitor 24, when VBRIGHT is high, determines the duty cycle of a high frequency PWM signal controlling a MOSFET switch 26. The PWM signal will typically be an HF signal between a few hundred KHz and a few MHz to keep component sizes and voltage ripple small. If VFB is too high, charge is removed from capacitor 24 to lower the capacitor 24 voltage. If VFB is too low, charge is added to capacitor 24 to increase the capacitor 24 voltage.
An oscillator 30 generates a sawtooth pattern at a fixed frequency, such as 1 MHz. A PWM comparator 32 compares the ramped oscillator signal to the capacitor 24 voltage. When the ramp cycle begins, the PWM output of the comparator 32 is high, turning on the switch 26. When the ramp voltage crosses the capacitor 24 voltage, the output of the PWM comparator 32 goes low, shutting off the switch 26. The resulting duty cycle of switch 26 maintains the inputs to op amp 22 approximately equal when VBRIGHT is high.
A MOSFET driver 36 receives the HF PWM signal and level-shifts the PWM signal, if necessary, to turn the switch 26 on and off in accordance with the PWM signal. While the switch 26 is generating HF pulses, a smoothing capacitor 38 maintains the VOUT and the current through the LEDs relatively constant. An inductor L1 and diode D0 operate in a well known manner to charge the capacitor 38 to a voltage above the power supply input voltage VDD and supply current to the LEDs during each switching cycle.
A capacitor 40 smoothes the power supply voltage VDD applied to the VIN pin of the integrated circuit LED driver 10. An external ENABLE signal, coupled to the EN pin of the LED driver 10, can be used to enable or disable the switch 26.
The external PWM low frequency brightness control signal 16 adjusts the brightness level of the LEDs D1-D4 by adjusting the overall duty cycle of the boost regulator. The frequency of the LF_PWM signal may be on the order of 100 Hz or higher to prevent noticeable flicker of the LEDs. There may be thousands of HF pulses generated by the boost regulator during each pulse (on-time) of the LF_PWM signal. When the low frequency signal LF_PWM goes low and the HF pulses are stopped, VOUT drops below the forward voltage of the LEDs volts to turn off the LEDs until the LF_PWM signal goes high again. In this way, the current through the LEDs is either a peak current or zero current. This keeps the emitted color of the LEDs constant, but enables the perceived brightness to be adjusted by the LF_PWM duty cycle.
If one of more of the LEDs becomes an open circuit, the forward current through the LEDs disappears, and therefore VFB will drop to ground level. The regulator's feedback loop will interpret this as a drop in VOUT. The op amp 22 will source more current (increase the error signal), causing the boost regulator to increase the duty cycle of the HF_PWM signal to increase the VOUT. Since the VFB will never rise because of the open LED condition, VOUT will keep climbing (a runaway condition). If there were no overvoltage protection, the increasing voltage would damage components and become a safety hazard.
To prevent such VOUT runaway failure from happening, a typical solution is put a resistor divider (R42 and R43) between VOUT and ground, in parallel to the LEDs and R14. The divided voltage is applied to an input of a comparator 46 whose other input is connected to VREF, which sets the Over Voltage Protection (OVP) level. When an open LED condition occurs and the VOUT starts rising, it will eventually rise above the OVP level, at which point the comparator 46 output will be asserted. The asserted output of comparator 46 will then shut down the MOSFET driver 36, causing the switch 26 to shut off and VOUT to decrease as the charge in the capacitor 38 is discharged through the LEDs and through R42 and R43.
There are three problems with the above typical solution:
1. The Over Voltage Protection (OVP) does not provide timely protection. This is because when an open LED condition occurs, the VOUT will still have to rise to the predetermined OVP level to activate the OVP circuit. This could be a long delay. By the time the switch 26 is finally turned off, VOUT may have already risen to a voltage that is higher than the OVP level and caused some irreversible damage.
2. If one or more LEDs shorts out, the forward voltage increases through the remaining LEDs, and the current increases. Due to the feedback loop, VOUT will be regulated downward to keep a predetermined current flowing through the LEDs. However, the LED driver 10 does not detect that certain LEDs are not longer operating, so the brightness level remains reduced.
3. The LED brightness change is not timely due to the delay incurred by the boost regulator being effectively enabled and disabled by the brightness control signal 16. The delay introduced causes brightness change delays between command and result and effectively causes the duty cycle of the LF_PWM signal to not match the duty cycle of the LEDs.
What is needed is an LED driver that does not have the above drawbacks of conventional LED drivers.